FIG. 1 shows the circuit diagram of a known oscillator. Similar oscillators are described, for example, from page 695 to page 698 of Analog Integrated Circuits, P. R. Gray and R. G. Meyer, published by Wiley, third edition, 1993, which is hereby incorporated by reference. This type of oscillator can be produced in an integrated circuit, and its frequency can be controlled by varying a voltage at one of its control terminals.
The circuit of FIG. 1 is formed by two branches, each of which comprises an NPN transistor Q1 and Q2. The collectors of the transistors Q1 and Q2 are connected to a first supply terminal Vcc through a respective resistor R1, R2 and the emitters thereof are connected to a second supply terminal, indicated by the ground symbol, through respective controllable current generators G1 and G2. The generators G1 and G2 are formed by two NPN transistors, the emitters of which are connected to ground through respective resistors and the bases of which are together connected to a voltage control terminal Vct. The emitters of the transistors Q1 and Q2 are coupled to one another by means of a capacitor C and the collectors thereof are each coupled to the base of the other by means of a respective NPN transistor Q3 and Q4. More precisely, the collectors of the transistors Q3 and Q4 are connected to the terminal Vcc, and the bases thereof are connected to the collectors of Q1 and Q2, respectively, and the emitters thereof are connected to the bases of Q2 and Q1, respectively, and also to the ground terminal through a constant current generator G4 and G3, respectively. A diode Q5, Q6 biased in the direction of conduction is connected in parallel with respective resistor R1, R2.
As is known, the circuit described operates as a free-running multivibrator in which the two transistors pass alternately from the conductive state to the non-conductive state as a result of the positive reaction between them, at a frequency determined by the capacity of the capacitor C and by the current of the generators G1 and G2. If the circuit is symmetrical and if the current of the generators G1 and G2 is large enough to bring about sufficient voltage drops in the resistors R1 and R2 to exceed the threshold voltage of the associated diodes Q5 and Q6, in the collectors of the transistors Q1 and Q2, that is to say, in the circuit terminals, voltage signals will be produced which are formed by complementary square waves of frequency EQU f=I/4C V.sub.BE(on) ( 1)
where I is the current of the generators G1 and G2, C is the capacitance of the capacitor indicated with the same symbol in FIG. 1 and V.sub.BE(on) is the voltage drop at a forward biased junction (it is assumed, to the first approximation, that the voltage drops at the base-emitter junctions of the transistors and at the junctions of the diodes are all equal).
The circuit lends itself to being used as an oscillator of which the frequency can be controlled by regulating the current of the current generators G1 and G2, in this case by applying a suitable control voltage to the terminal Vct. The voltage variations at the emitter of Q2 are shown in the graph of FIG. 4.
The frequency of the oscillator is directly proportional to the current I in accordance with the relationship shown in equation (1) above provided that, as was indicated above, the current is sufficiently large to permit forward conduction through the diodes Q5 and Q6. If it is not, the frequency depends on the voltage drop across the resistors R1 and R2 and its dependence on the current I becomes inversely proportional. There is thus a non-monotonic characteristic of the frequency as a function of the current, as shown by the curve shown in FIG. 3. The minimum frequency of the oscillator is therefore determined by the value of the current at which the inversion of the gradient of the characteristic takes place. The above-described behavior must absolutely be avoided if the oscillator is to be used in a phase-locked loop (PLL), because the lack of monotonicity could give rise to a positive feedback during the locking transient. The oscillator can therefore be used in this case provided that the current is prevented from falling below a predetermined value.
Next consider the circuit of FIG. 5, including the generating group Q7, Q8 and GS. In the circuit of FIG. 5, components that are the same as those of the circuit of FIG. 1 are indicated by the same reference symbol. This circuit differs from that of FIG. 1 basically by the fact that it comprises current generating means associated with the two transistors Q1 and Q2. It also has non-substantial differences owing to the fact that, in parallel with the resistors R1 and R2, instead of the diodes Q5 and Q6, there are two npn transistors Q5' and Q6' which are connected as diodes and are biased in the direction of conduction and owing to the fact that its outputs, marked Vout', are on the bases, instead of on the collectors, of Q1 and Q2. The current generating means associated with the transistors Q1 and Q2 are formed by a constant current generator G5 and by two npn transistors Q7 and Q8 of which the emitters are together connected to ground through the generator G5 and the base and collector terminals are connected, repsectively, to the base and collector terminals of the associated transistors Q1 and Q2.
Consider the operation of the oscillator of FIG. 5.
It will be assumed that Q1 is conductive and that Q2 is non-conductive. The transistor Q5' is conductive if the following condition for the base and emitter voltages V.sub.BQ5' and V.sub.EQ5 ', respectively, is satisfied: EQU V.sub.BQ5' -V.sub.EQ5' .gtoreq.V.sub.BE(on) Q5' (5)
where
V.sub.BE(on)Q5' is the voltage across the forward-biased base-to-emitter junction of Q5', its base voltage is:
V.sub.BQ5' =Vcc PA1 V.sub.EQ5' =Vcc-IR PA1 IR&gt;V.sub.BE(on)Q5'
and its emitter is:
wherein I is the current of the generator G1 and R is the resistance of the resistor R1.
By substituting into (5) there is obtained
which is the condition for Q5' to conduct.
In the oscillator of FIG. 1, for values of the current I that are too low, so that the drop in R1 is less than V.sub.BE(on)Q5 ', the frequency of the oscillator is inversely proportional to the current, as is shown by FIG. 3.
The transistors Q7 and Q8 (compensation transistors) are brought into conduction or non-conduction simultaneously with the transistors Q1 and Q2 with which they are associated; they therefore inject the current Icomp of the generator G5 alternately into one or other resistor R1 or R2 when the respective transitor Q1 and Q2 is conductive. The current Icomp is selected in such a manner that the voltage drop across one or other resistor R1, R2, due to the sum of the current I of one or other generator G1, G2 and the current Icomp, is at least equal to the voltage V.sub.BE(on)Q5'. The frequency of the oscillator is thus not governed by the voltage drop in R1 or R2 and can be regulated to very low values by establishing correspondingly low control currents. In addition, the frequency-current characteristic is monotonic, as illustrated by FIG. 6, and the oscillator can therefore be used in PLL loops without limiting the synthesizable frequencies and without any risk of positive feedback during the locking transients.
It is of course also necessary for the current Icomp not to be so high as to maintain the transistors, connected as diodes Q5' and Q6', in the conductive state even without the contribution of the currents I of the generators G1 and G2, otherwise the circuit would not be able to oscillate.
By way of example, the most important parameters of an oscillator produced in practice, having the charactersitic of FIG. 6, were the following: Vcc=5 V, R=Ohm, C=1 pF, Icomp=100 .mu.A.
FIG. 2 shows another known oscillator embodiment, which differs slightly from the circuit of FIG. 1. Note that the output of the circuit of FIG. 2 is dependent on the value of Vbe, i.e. the voltage drop at a forward-biased junction, which may vary according to changes in process or temperature. Such variance is, of course, undesirable.
The frequency of the oscillator of FIG. 2 is determined by the value of Vbe and the value of the current Idr (I data rate): .function.=Idr/(4*V.sub.be *Ctiming)
Therefore, the oscillator frequency will vary according to changes in Vbe, which may be caused by changes in temperature and process.
Therefore there is disclosed an innovative circuit for an oscillator which remains accurate independent of changes in process or temperature.